Add your blueberries to a small bowl and add in the sugar, flour, and salt. Mix to coat each of the berries and set aside.

Let your puff pastry thaw out (if frozefor 40-50 minutes on a cutting board. Once thawed, open it up and lay it flat - my package came with 2 sheets. Cut each sheet into 9 square sections, making 18 total. (I baked 12 of these at a time, having 6 left over for the next set to bake)

With your muffin tin, spray the tin lightly with cooking spray to prevent the tarts from sticking. Place 1 of the puff pastry squares into the muffin tin section and press it in. Use your fingertips or a fork to add small holes to the center of the pastry to help it avoid forming a bubble.

Add a spoonful of blueberry mix to each of the puff pastry sections, about half full of berries.

Add one small square of butter to each of the tarts, and sprinkle one more time with a little sugar - optional.

Bake your tarts at 400 degrees for 15-16 minutes or until the edges are golden brown. Let them cool on a rack for 10 minutes.

You can eat them as is or even add a little dollop of whipped cream or ice cream. Enjoy!